Test and measurement instruments such as logic analyzers and mixed signal oscilloscopes are used to measure the logic values of digital logic signals. These instruments sample the logic values of an input digital logic signal at time instants specified by a sampling clock, store the logic samples in an acquisition memory, convert the stored logic samples into a waveform image in display memory, and display the waveform image on a display device.
One limitation of these instruments is their finite acquisition memory capacity. The acquisition memories are standard commercially available random access memories and, as such, are available in fixed capacities such as 64, 128, or 256 Mbytes. For example, the TLA7000 Series Logic Analyzer available from Tektronix, Inc. of Beaverton, Oreg. supports up to 256 Mbytes of acquisition memory.
Users are occasionally interested in observing the behavior of the input digital logic signal over a time interval that exceeds the instrument's acquisition memory capacity. For example, consider a user interested in the behavior of a signal over three seconds, but the user's instrument is acquiring logic samples at 256 Mbytes/sec and has 256 Mbytes of acquisition memory. To display three seconds requires 3×256 Mbytes=768 Mbytes of acquisition memory, three times more than is available. In this situation, instruments typically “decimate” or discard some of the acquired samples to avoid overflowing the memory. For example, the instrument may decimate by a factor of three, storing only one of every three acquired samples and discard the remaining two. The resulting display is “aliased” in that it no longer contains all the sample data, but nonetheless it still provides useful information to the user. However, if the discarded samples contain important information about the signal (e.g. logic transitions) the decimated display misleads the user.
Decimation is a very simple compression method, with the loss of information being a potential user trap. U.S. Pat. No. 6,473,700 to Holaday et al. for an “Apparatus for Use in a Logic Analyzer for Compressing Digital Data for Waveform Viewing” describes a more elaborate compression method. Holaday teaches conditioning a large number of logic samples stored in acquisition memory, for example, 256 Mbytes, for display on a raster scan type display with, for example, 1024 columns. Holaday's compression method parses the logic samples into groups (e.g. 256 Mbytes÷1024=250 k bytes per display column) and assigns each group a compression code based on whether the logic samples in the group were “always high”, “always low”, or “changed.” Holaday's compression method, unlike decimation, does not alias information. That is, in compressing 250 k samples into one display column Holaday does not preserve all of the details of the logic activity, but if, for example, a region contains logic transitions, Holaday accurately reports that the signal “changed,” as opposed to decimation which may show that the signal did not change. One might be led to think that Holaday could be applied in place of decimation to solve the memory capacity problem. However, because Holaday's purpose is to condition logic samples already stored in acquisition memory for display on a raster scan display device, Holaday relies on memory addresses corresponding to stored logic samples, and applies those memory addresses to comparators in order to parse the logic samples. In order to alleviate the problem of limited acquisition memory capacity, the logic samples must be compressed before they are stored in acquisition memory, and so Holaday is not applicable.
What is needed is a compression method capable of operating on logic samples as they are acquired, in real time, before they are stored in acquisition memory, thereby allowing an instrument to store more information about an input digital logic signal than finite memory capacity ordinarily allows, without losing important information about the signal activity.